1. Field of the Invention
The present invention relates to an optical beam deflector. The optical beam deflector can be used, for example, for generating a scanning beam in a laser beam scanning apparatus, and the laser beam scanning device can be used in recording, reading, and displaying information, laser radar, intersatellite communication, and the like.
2. Description of the Related Art
Conventionally, the following techniques are proposed for optical beam deflectors:
(1) Laser Beam Deflectors Using Microlens Array
(a) W. Goltsos, and M. Holz, xe2x80x9cAgile Beam Steering Using Binary Optics Microlens Arrays,xe2x80x9d Optical Engineering, vol. 29 (1990), 1392.
(b) T. D. Milster and J. N. Wong, xe2x80x9cModeling and Measurement of Micro-Optic Beam Deflector,xe2x80x9d in xe2x80x9cDesign, Modeling, and Control of Laser Beam Optics,xe2x80x9d SPIE Proceedings, vol. 1625 (1992), 78-83.
In the techniques disclosed in the above references (a) and (b), two microlens arrays are provided in a telescope arrangement, and an incident laser beam is deflected by displacing one of the two microlens arrays in the direction perpendicular to the light axis. However, these techniques have the following drawbacks.
(i) Distributions of amplitudes and phases of the deflected beam are inequable, i.e., the deflected beam is not a single-peaked beam. Luminous energy of portions of the laser beam which are diffracted in directions other than the desired direction of deflection is lost, quality of the light beam is deteriorated, and performance of apparatuses utilizing the optical beam deflector is impaired.
(ii) Since the deflection is realized by mechanical displacement of the microlens array, it is not possible to perform agile, stable, and reliable scanning by using the above optical beam deflectors.
(2) Laser Beam Deflectors Using Liquid Crystal Phase Modulator Array
R. M. Matic, xe2x80x9cBlazed Phase Liquid Crystal Beam Steering,xe2x80x9d SPIE Proceedings, vol. 2120 (1994), 194-205.
In the technique disclosed in the above reference, an incident laser beam is deflected in a desired direction by providing a distribution of phase modulation over the crosssection of the laser beam by using an array of optical phase modulators. However, due to use of only one array of optical phase modulators, this technique has the following drawbacks.
(i) Dead spots or stripe-shaped electrode areas of the array of optical phase modulators cause loss of luminous energy, and deform the wavefront of an output laser beam.
(ii) The maximum deflection angle is determined by a maximum gradient of the phase distribution, which is further determined by a pitch of the electrodes and the maximum phase difference achieved by each optical phase modulator in the array of optical phase modulators. Therefore, in order to increase the maximum deflection angle, the maximum gradient of the phase distribution has to be increased. That is, it is necessary to decrease the pitch of the electrodes, or to increase the maximum phase difference achieved by each optical phase modulator.
However, when the pitch of the electrodes is decreased, a considerable amount of phase distortion is generated at edge portions of the array of optical phase modulators, and it becomes impossible to realize a desired stepwise phase distribution. In addition, in order to increase the above maximum phase difference achieved by each optical phase modulator, it is necessary to thicken each optical phase modulator. However, when each optical phase modulator is thickened, a large driving voltage is needed, and response becomes slow.
Therefore, it is difficult to increase the maximum deflection angle with the above technique.
(3) Laser Beam Scanning Radiating Apparatus Using Laser Resonator
U.S. Pat. No. 5,600,666 discloses a laser beam scanning and radiating apparatus which generates and scans a laser beam by using a laser resonator.
However, in the above laser beam scanning and radiating apparatus, resonator mirrors must be a phase conjugate mirror. Nevertheless, usually, a passive optical element such as a mirror can achieve the optical phase conjugation only approximately. Therefore, the use of the resonator mirrors causes loss of luminous energy. In addition, the laser beam scanning and radiating apparatus using a laser resonator has a complex construction.
Further, active elements such as nonlinear optical crystals are expensive, and cannot realize a stable phase-conjugate element.
The object of the present invention is to provide an optical beam deflector which causes neither loss of luminous energy nor deformation of wavefront, has no limit of the maximum deflection angle, guarantees agile, stable, and reliable movement of a beam, has a simple construction, and is inexpensive.
According to the present invention, a crosssection of an incident optical (e.g., laser) beam is divided into a plurality of first micro crosssections, a plurality of phases of the optical beam in the first micro crosssections are respectively modified so that a desired phase gradient is realized over the crosssection of the optical beam, and thereafter the crosssection of the optical beam is further divided into a plurality of second micro crosssections, and a plurality of phases of the optical beam in the second micro crosssections are respectively modified so that the optical beam as a whole is finally directed in a desired direction of deflection or to a desired point.
More concretely, the optical beam deflector according to the first aspect of the present invention contains an array of lenses which collect a first plurality of portions of an incident optical beam into a plurality of spots, respectively; an array of first optical phase modulators which modulate, at or in vicinities of the plurality of spots, phases of the first plurality of portions of the optical beam; a first Fourier transform lens which performs Fourier transformation on the optical beam output from the array of first optical phase modulators to generate a second plurality of portions of the optical beam; an array of second optical phase modulators which modulate phases of the second plurality of portions of the optical beam; an array of second Fourier transform lenses which collect the second plurality of portions of the optical beam after the phases of the second plurality of portions are modulated by the array of second optical phase modulators, and perform inverse Fourier transformation on the second plurality of portions of the optical beam; and a driving unit which drives the arrays of first and second optical phase modulators so that the second plurality of portions of the optical beam which exit from the array of second Fourier transform lenses as a whole are directed in a desired direction of deflection or to a desired point.
In the optical beam deflector according to the first aspect of the present invention, a first plurality of portions of an incident optical beam are collected, by the array of lenses, respectively into a plurality of spots. The phases of the first plurality of portions of the optical beam are modulated by the array of first optical phase modulators at or in vicinities of the plurality of spots. Fourier transformation is performed by the first Fourier transform lens on the optical beam output from the array of first optical phase modulators, to generate a second plurality of portions of the optical beam. The phases of the second plurality of portions of the optical beam are modulated by the array of second optical phase modulators. The second plurality of portions of the optical beam, after the phases of the second plurality of portions are modulated by the array of second optical phase modulators, are collected by the array of second Fourier transform lenses, and inverse Fourier transformation is performed on the second plurality of portions of the optical beam. The driving unit drives the arrays of first and second optical phase modulators so as to realize appropriate phase distributions over the arrays of first and second optical phase modulators, respectively. Thus, the second plurality of portions of the optical beam which exit from the array of second Fourier transform lenses as a whole are directed in a desired direction of deflection or to a desired point.
Preferably, in the above construction of the first aspect of the present invention, the array of lenses, the first Fourier transform lens, and the array of second Fourier transform lenses may be arranged so that the back focal planes of the array of lenses and the front Fourier transform plane of the first Fourier transform lens coincide, and the back Fourier transform plane of the first Fourier transform lens and the front Fourier transform planes of the array of second Fourier transform lenses coincide. In addition, preferably, the array of first optical phase modulators may be located on the back focal planes of the array of lenses, and the array of second optical phase modulators may be located on the front Fourier transform planes of the array of second Fourier transform lenses.
Further, when p1 is the pitch of the array of lenses, N1 is the number of the lenses in the array, f is the focal length of the first Fourier transform lens, f2 is the focal length of the second Fourier transform lenses, p2 is the pitch of the array of second Fourier transform lenses, and xcex is the wavelength of the optical (laser) beam, it is preferable that the above construction satisfies the following equations (1) and (2).
fxc3x97p2 =f2 xc3x97p1xc3x97N1xe2x80x83xe2x80x83(1)
(p1xc3x97p2)/(xcexxc3x97f)=1xe2x80x83xe2x80x83(2)
On the other hand, the optical beam deflector according to the second aspect of the present invention does not contain the above first Fourier transform lens and the array of second Fourier transform lenses, and contains an array of second (image-forming) lenses, instead of the array of second Fourier transform lenses in the first aspect of the present invention.
The optical beam deflector according to the second aspect of the present invention contains an array of first lenses which collect a first plurality of portions of an incident optical beam into a plurality of spots; an array of first optical phase modulators which modulate, at or in vicinities of the plurality of spots, phases of the first plurality of portions of the optical beam; an array of second optical phase modulators which modulate phases of a second plurality of portions of the optical beam output from the array of first optical phase modulators; an array of second lenses which collect the second plurality of portions of the optical beam output from the array of second optical phase modulators; and a driving unit which drives the arrays of first and second optical phase modulators so that the optical beam which exits from the array of second lenses as a whole is directed in a desired direction of deflection or to a desired point.
In the optical beam deflectors according to the second aspect of the present invention, a first plurality of portions of an incident optical beam are collected, by the array of lenses, respectively into a plurality of spots. The phases of the first plurality of portions of the optical beam are modulated by the array of first optical phase modulators at or in vicinities of the plurality of spots. The phases of a second plurality of portions of the optical beam output from the array of first optical phase modulators are modulated by the array of second optical phase modulators. The second plurality of portions of the optical beam output from the array of second optical phase modulators are collected by the array of second lenses. The driving unit drives the arrays of first and second optical phase modulators so as to realize appropriate phase distributions over the arrays of first and second optical phase modulators. Thus, the second plurality of portions of the optical beam which exit from the array of second lenses as a whole are directed in a desired direction of deflection or to a desired point.
Preferably, in the above construction of the second aspect of the present invention, the array of first optical phase modulators may be located on the back focal planes of the array of first lenses, and the array of second optical phase modulators may be located on the front focal planes of the array of second lenses.
The optical beam deflectors according to the first and second aspects of the present invention realize deflection without loss of luminous energy and deformation of the wavefront due to dead spots or stripe-shaped electrode areas of the array of optical phase modulators, and enable agile, stable, and reliable scanning. In addition, it is possible to achieve a greater maximum deflection angle than that achieved by the conventional technique since the maximum total phase difference is determined by the numerical aperture (NA) of the array of second Fourier transform lenses (in the first aspect) or the numerical apertures of the array of second lenses (in the second aspect), and depends on neither the maximum phase difference achieved by each optical phase modulator nor the pitches of the arrays of optical phase modulators or Fourier transform lenses. Further, the construction of the optical system is simple and the cost is low since the laser resonator using resonator mirrors having the function of phase conjugation is not used.
In the constructions of the first and second aspects of the present invention, when the incident optical beam is a coherent optical beam such as a laser beam, the driving unit drives the above arrays of first and second optical phase modulators so that wavefronts of the second plurality of portions of the optical beam output from the array of second Fourier transform lenses or second lenses have an identical phase with respect to the above desired direction of deflection or to the above desired point.
In the above case, the incident laser beam may be any kind of laser beam, and the first and second optical phase modulators may be any kind of optical phase modulators.
In addition, when the driving unit appropriately changes the deflection angle of the optical beam output from the optical beam deflector according to the first and second aspects of the present invention, the optical beam can be used as a scanning beam. That is, a scanning apparatus can be realized using the optical beam deflector according to the first and second aspects of the present invention.
Further, when intensity of the optical beam is modulated in the above scanning apparatus, it is possible to change the intensity of the scanning beam.